The TAG (Trans-Atlantic Geotraverse) hydrothermal field (26 degrees 08'N on the Mid-Atlantic Ridge) is one of the largest and best-studied sites of high-temperature hydrothermal activity and mineralization that has been found to date on the seafloor. The assemblage of active and relict deposits, with ages ranging from 0 to 140 ka, attests to a long, complex and episodic history of hydrothermal activity that appears to be controlled dominantly by large-scale faulting associated with the formation of the eastern wall of the median valley. The alternation between periods of activity and quiescence may reflect either episodic magmatism and the replenishment of the heat source or the occurrence of faulting events that reactivate fluid-flow pathways. The construction of large circular mounds, rather than the elongate deposits commonly seen in ophiolites, requires focusing of fluids along discrete pipe-like up-flow zones. Geochemical and isotopic studies of the TAG active mound and underlying shallow up-flow zone provide strong evidence that variable amounts of entrainment of seawater play a key role in the generation of distinct fluid chemistries and the minerals that form the precipitates, the alteration of the basalts, and the growth and structural evolution of the entire deposit. We present a model for the formation and growth of the TAG active mound in which the current activity is dominated by seawater entrainment and precipitation of anhydrite due to the vigorous and focused nature of the current black smoker activity. We suggest that the pervasive, high-temperature alteration of the basement in the shallow up-flow zone likely occurs during either waxing or waning of activity, when subsurface fluid-flow rates are substantially lower and seawater entrainment is not as prevalent.